1. Field of the Invention
The present invention relates to methods of fabricating channel plates for ink jet printers having side edges that are precisely located relative to the channels formed therein, and more particularly to methods of forming ink jet printheads incorporating such channel plates.
2. Description of Related Art
Thermal ink jet printing systems use thermal energy selectively produced by resistors located in capillary filled ink channels near channel terminating nozzles or orifices to vaporize momentarily the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium. The printing system may be incorporated in either a carriage type printer or a pagewidth type printer. The carriage type printer generally has a relatively small printhead, containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is reciprocated to print one swath of information at a time on a stationarily held recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. For an example of a carriage type printer, refer to U.S. Pat. No. 4,571,599 to Rezanka. In contrast, the pagewidth printer has a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the pagewidth printhead in a direction normal to the printhead length and at a constant speed during the printing process. Refer to U.S. Pat. No. 4,829,324 to Drake et al for an example of pagewidth printing and especially FIGS. 10, 11 and 13 therein.
U.S. Pat. No. 4,829,324 mentioned above discloses a printhead having one or more ink filled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth of the bubbles which causes a quantity of ink to bulge from the nozzle and break off into a droplet at the beginning of the bubble collapse. The current pulses are shaped to prevent the meniscus from breaking up and receding too far into the channels after each droplet is expelled. Various embodiments of linear arrays of thermal ink jet devices are shown, such as those having staggered linear arrays attached to the top and bottom of a heat sinking substrate for the purpose of obtaining a pagewidth printhead. Such arrangements may also be used for different colored inks to enable multi-colored printing.
In order to achieve higher output rates, it is desirable to use pagewidth printheads, such as disclosed in the above-mentioned U.S. Pat. No. 4,829,324. A preferred method of fabricating these pagewidth printheads involves butting a plurality of printhead subunits against one another to form an array of subunits having a length corresponding to the width of a page. It is desirable to use this subunit approach because it is difficult to form a monolithic printhead having a length corresponding to the width of a page. Monolithic printheads having the length of a pagewidth are also not preferred because one defective channel or heating element out of the 2,550 channels or heating elements contained in a monolithic printhead would render the entire printhead useless. In the subunit approach, each discrete subunit can be tested prior to assembly and thus only subunits are discarded if they contain a defective channel or heating element.
The printhead subunits are typically made from a heater plate which includes a plurality of resistive elements or heater elements formed on an upper surface thereof and a channel plate having a plurality of channels, corresponding in number and position to the heater elements, formed on a base surface thereof. The upper surface of the heater plate is bonded to the base surface of the channel plate so that a heater element is located in each channel. The channel plate can also include a fill hole, extending from its upper surface to its base surface, which is in fluid communication with the channels so that ink supplied from an ink source flows into the channels. The heater plate and channel plate are typically formed in separate (100) silicon wafers which are bonded to one another and diced with, for example, a precision dicing saw to form discrete printhead subunits. This technique lends itself to mass production of discrete printhead subunits because a plurality of heater plates can be formed in horizontal rows and vertical columns in one silicon wafer, a plurality of channel plates can be formed in horizontal rows and vertical columns in another silicon wafer, and the wafers can be bonded to each other and diced between each horizontal row and vertical column to form a plurality of printhead subunits. See, for example, previously cited U.S. Pat. No. 4,829,324 to Drake et al, the disclosure of which is herein incorporated by reference, which shows a plurality of arrangements of heater plates and channel plates for forming pagewidth printheads. In particular, FIGS. 12 and 13 show an embodiment wherein an etched silicon channel plate wafer is aligned and bonded to a silicon heater plate wafer after which the sandwiched wafers are diced to form multiple printhead subunits.
After dicing a wafer sandwich into multiple printhead subunits, a plurality of subunits are aligned in an array with the side edges or surfaces of adjacent subunits butting against one another and are bonded together to form, for example, a pagewidth printhead. One of the most critical parts of this task is in precisely delineating the discrete printhead subunits. Ink jet printhead subunit delineation is more difficult than standard chip dicing because the printhead subunit is a sandwich structure of two wafers: a heater plate wafer and a channel plate wafer. For example, FIG. 1 shows a first (100) silicon wafer 2 from which a plurality of channel plate subunits 4 are formed bonded to a second silicon wafer 20 from which a plurality of heater plate subunits 21 are formed. As seen from FIG. 1, a plurality of channel plate subunits 4 are formed in horizontal rows and vertical columns on (100) silicon wafer 2. In the example shown in FIG. 1, silicon wafer 2 need be only a one-side polished (100) silicon wafer which, after being chemically cleaned, has a silicon nitride layer (not shown) deposited on one side. Using conventional photolithography, vias for channel grooves 6 and fill holes 8 are printed on the silicon nitride layer. The silicon nitride is plasma etched off of the patterned vias representing the channel grooves 6 and fill holes 8 and a potassium hydroxide (KOH) anisotropic etch is used to etch channel grooves 6 and fill holes 8. In this case, the (111) planes of the (100) wafer make an angle of 54.7.degree. with the surface of the wafer. The vias for fill holes 8 are sized so that they are entirely etched through wafer 2, whereas the vias for channel grooves 6 only etch partially through the wafer 2 as illustrated in FIG. 2.
Referring to FIG. 2, channel wafer 2 is bonded to heater wafer 20 which includes a plurality of heater plates 21, each including a set of heater elements 22 thereon. Each heater element includes a resistor having a passivated addressing electrode (not shown) so that current pulses representative of data signals can be selectively applied to each resistor. Silicon wafers 2 and 20 are bonded to each other so that a single heater element 22 is located in each channel 6. Wafers 2 and 20 are properly aligned with each other by forming alignment openings (not shown) in each wafer prior to or during formation of channels 6 and heater elements 22 on each respective wafer 2, 20.
After wafers 2 and 20 are bonded to each other, the wafer sandwich is separated into discrete printhead subunits by dicing between each row and column of mated sets of channels 6 and heater elements 22. The rows are separated out of the wafer by dicing along lines 10 and 12 and the printheads are separated out of each row by dicing along lines 14 and 16. Dicing along line 10 forms open ends or nozzles for each channel 6. Dicing along lines 14 and 16 with dicing blade 24 forms the side surfaces of each printhead. The preciseness with which the side surfaces of each printhead subunit is formed is critical because it controls the accuracy with which adjacent subunits are butted. Dicing blade 24 must cut through both channel wafer 2 and heater wafer 20 in the example of FIG. 2. Cutting through both wafers reduces the useful life of the cutting blade and also reduces the precision with which discrete printhead subunits can be formed as compared to an arrangement whereby dicing through only one wafer, for example, the heater wafer, is required. In particular, blade 24 tends to bend uncontrollably with increased thickness through which blade 24 must dice. This bending of blade 24 during dicing creates non-uniform side surfaces on the printhead subunits and reduces the accuracy with which subunits can be butted against each other.
U.S. Pat. No. 4,851,371 to Fisher et al, the disclosure of which is herein incorporated by reference, discloses a method of fabricating printhead subunits from a channel wafer/heater wafer sandwich wherein elongated slots 26 (see FIG. 3 of the present application) are formed on both ends of each set of channel grooves 6 for each channel plate 4 formed on silicon wafer 2. Elongated slots 26 are located so that dice cuts 14 and 16 which separate the printheads out of each row of printheads pass through grooves 26. As illustrated in FIG. 4, this construction permits printhead subunits to be formed wherein dicing blade 24 only dices through heater plate wafer 20 along the side surfaces of each subunit. Thus, elongated groove 26 reduces the thickness of silicon through which dicing blade 24 must cut, consequently increasing the usable life of dicing blade 24 and reducing the amount of uncontrollable blade bending to enable the production of precisely delineated printhead subunits. In the fabrication process of U.S. Pat. No. 4,851,371, channel wafer 2 is a two-side polished (100) silicon wafer which is cleaned and coated with a silicon nitride layer on both sides. Vias for elongated slots 26 are formed on one side of silicon wafer 2 while vias for channel groove 6 and fill holes 8 are formed on the opposite side of silicon wafer 2. As described above, the silicon nitride is plasma etched off of the pattern vias representing elongated slots 26, channel grooves 6 and fill holes 8 and the wafer 2 is then anisotropically etched by KOH to form a plurality of channel plates 4 in silicon wafer 2.
A problem fabricating this cross-sectional structure is that simple orientation dependent etching (ODE) through holes in commercial silicon wafers is not precise enough. This is because the dimensions of a through etch opening are a function of the wafer thickness as shown in FIG. 5. As discussed above, elongated slots 26 are etched through silicon wafer 2 from a side of silicon wafer 2 opposite from the side in which channel grooves 6 are formed. FIG. 5 illustrates that the location of the side edge 28 of each channel plate is controlled by the size of the opening O.sub.1, O.sub.2 that is formed by elongated slot 26 on the surface of silicon wafer 2 which contains channel grooves 6. As can be seen, the size of the opening O.sub.1, O.sub.2 is a function of the thickness t.sub.1, t.sub.2 of silicon wafer 2. As the thickness of silicon wafer 2 increases from t.sub.2 to t.sub.1, the size of the opening on the channel groove side of silicon wafer 2 decreases from O.sub.2 to O.sub.1. Consequently, since channels 6 are patterned on a side of wafer 2 opposite from the side on which channel groove 26 is formed, the distance which the end channel groove 6 is located from side edge 28 of channel plate 4 will increase from d.sub.2 to d.sub.1 with increased wafer thickness. This variation in distance d.sub.1, d.sub.2 of the end channel plate 6 from the side edge 28 of each channel plate 4 results in non-uniform spacing of the end channel grooves 6 between adjacent printhead subunits in an extended array (i.e. a pagewidth array) of printhead subunits. Additionally, if the thickness t.sub.1 of silicon wafer 2 is excessively large, a portion of side edge 28 of channel plate 4 may extend into the dicing zone of dicing blade 24 and thus defeat some of the primary advantages of this fabrication method.
U.S. Pat. No. 4,822,755 to Hawkins et al also discloses the use of elongated slots in a (100) silicon wafer from which a plurality of channel plates are fabricated to form side edges of the channel plates. However, as with the above-mentioned U.S. Pat. No. 4,851,371, the end channels in each channel plate cannot be accurately located relative to the side edges of each channel plate due to variations in the thickness of the silicon wafers from which channel plates are fabricated.